Inconel 690 is a high-chromium nickel alloy (approximately 60 percent nickel, 30 percent chromium, 9 percent iron). The high chromium content provides exceptional resistance to oxidizing environments and to high-temperature corrosion, particularly stress corrosion cracking in chloride and caustic conditions. It also offers excellent resistance to nitric acid and other oxidizing media.

The nominal chemical composition is: nickel 58.0 to 65.0 percent, chromium 27.0 to 31.0 percent, iron 7.0 to 11.0 percent, carbon up to 0.05 percent, manganese up to 0.50 percent, and silicon up to 0.50 percent. The low carbon content improves weldability and reduces carbide precipitation.

Key mechanical properties (typical, annealed condition):

● Tensile strength: 580 to 690 MPa

● Yield strength (0.2% offset): 240 to 350 MPa

● Elongation: 35 to 50 percent

● Hardness: 70 to 90 HRB

● Density: 8.19 g/cm³

Inconel 690 exhibits outstanding resistance to chloride and caustic stress corrosion cracking, superior to Inconel 600 and 625 in many environments. It also offers excellent oxidation resistance at high temperatures (up to about 1,100°C) and good resistance to sulfidation and carburization.

Common standards: ASTM B168 (plate, sheet, strip), B166 (rod, bar, wire), B167 (pipe, tube), ASME SB-168, SB-166, SB-167, and AMS 5878 (sheet).

Typical applications include:

● Nuclear power: steam generator tubing in pressurized water reactors (PWRs), replacing Inconel 600 due to its superior resistance to primary water stress corrosion cracking.

● Chemical processing: equipment handling nitric acid, caustic solutions, and high-temperature oxidizing environments.

● Heat treatment: furnace components, muffles, and radiant tubes.

● Waste treatment: incinerator components and flue gas desulfurization systems.

Inconel 690 is often compared to Inconel 600 (lower chromium, less SCC resistance) and Inconel 625 (higher molybdenum, better pitting resistance but lower chromium). For nuclear steam generator tubing, Inconel 690 is the modern standard. For general high-temperature oxidation and caustic service, it is an excellent choice.